A digital TV broadcaster, adhering to the Digital Video Broadcast specification, broadcasts pictures using a program clock reference (PCR) based on a 27 MHZ clock frequency, with an error of +/−810 Hz. To accurately reproduce the broadcast program within a receiver, the same clock frequency must be generated within the receiver using a local clock oscillator. Samples of a broadcaster's (PCR) are broadcast at intervals to facilitate frequency synchronization of the local clock oscillator. Failure to maintain synchronization at the receiver would result in some audio and video data being lost (never presented) or repeated (as the receiver waits for more).
A conventional approach to implementing the local clock oscillator is to use a voltage controlled crystal oscillator (VCXO). Control logic in the receiver processes received samples of the PCR, and processes values derived from a local clock counter clocked by the local clock oscillator, to determine whether the local clock frequency is synchronized to the PCR. If not, then a control voltage applied to the VCXO is adjusted to either increase, or decrease, the VCXO frequency, to thereby increase the counting rate of the local clock counter (if the counting rate is slightly slower than the received PCR samples), or to decrease the counting rate of the local clock counter (if the counting rate is slightly faster than the received PCR samples).
However, a VCXO circuit is generally difficult to design, due to the large number of variables within its analog circuitry. Guaranteeing a predetermined frequency operating range of the VCXO is difficult. For the above reason, the VCXO is generally separate from, and not integrated into, a decoder integrated circuit. Furthermore, the VCXO represents a significant cost within the circuitry of the receiver. The cost becomes even more significant in a receiver with a multiple (i.e., dual) broadcast reception capability. Each channel may include a slightly different PCR frequency, and so corresponding multiple local clocks have to be generated and synchronized, one for each broadcast channel to be received or decoded. A separate VCXO has to be employed for each channel, to provide an independently controllable clock for that channel.